The invention relates to a tire with radial carcass reinforcement anchored in each bead to at least one bead wire, and comprising a crown reinforcement formed by at least two so-called working plies, superposed and formed of reinforcing elements which are parallel to each other in each ply and are crossed from one ply to the next, forming angles of at most 40.degree. in absolute value with the circumferential direction of the tire.
U.S. Pat. No. 4,688,615 describes a crown reinforcement for a radial tire composed of a first ply and of a second ply formed by cables parallel to each other in each ply, crossed from one ply to the next, forming an angle with the circumferential direction of the tire that can range between 5 and 60.degree.. A third ply of circumferentially arranged cables is placed between said two plies, those cables having a diameter at most equal to the diameter of the cables of the first and second plies, and being made of a material having a tensile strength less than the tensile strength of the material forming the cables of the first and second plies, which imparts a lesser tensile strength to said third ply, and being more extensible than each of the plies with crossed cables, said third ply being no wider than the ply of cords with the widest angle.
French Application FR 94/15,736, corresponding to U.S. Pat. No. 5,738,740 relates to a tire as described above, and more particularly to a heavy vehicle tire, the ratio of the height above rim H to the maximum axial width S of which is at most 0.60. Said application, in order to improve the life of the crown reinforcement of such a tire, and also the regularity of wear of the tread thereof, requires an architecture of the crown reinforcement characterized by the combined presence in said reinforcement of an axially continuous ply formed of inextensible metal reinforcing elements forming an angle of at least 60.degree. with the circumferential direction of the tire, and a ply of metallic elements oriented substantially parallel to the circumferential direction, arranged radially between the two working crown plies.
Such an architecture makes it possible to obtain better resistance to separation between working plies, and also better fatigue strength of the cables of the carcass reinforcement located beneath the crown reinforcement thus formed. The axial width of the additional ply of circumferential reinforcing elements may be less than the widths of the working plies.
Since the operating temperatures in the tires of form ratio of at least 0.50 are not negligible, Applicant's research has led him to look for a solution which is more effective from the thermic and economic points of view.
In order to improve the life of the crown reinforcement and subjacent carcass reinforcement of a tire, without using a crown ply formed by metal cables very greatly inclined relative to the circumferential direction of the tire and located radially above the carcass reinforcement, the present invention proposes a more economic solution than that described in the above French application.
The tire having an H/S form ratio of at least 0.50, according to the invention, comprises a radial carcass reinforcement and a crown reinforcement composed of at least two working crown plies made of inextensible metal cables, crossed from one ply to the next, forming angles of between 10.degree. and 45.degree. with the circumferential direction, of a protective crown ply formed of elastic metallic cables made of steel oriented relative to the circumferential direction at an angle of the same direction as the angle formed by the cables of the radially outermost working crown ply, and of an additional ply which is axially continuous and formed of metallic reinforcing elements made of steel oriented substantially parallel to the circumferential direction, the additional ply being arranged above the radially closest working ply to the carcass reinforcement, the presence of a crown ply formed of metallic elements oriented relative to the circumferential direction by an angle greater than 45.degree. being excluded, and is characterized in that the axial width of the protective ply is greater than the axial width of the additional ply of metallic element made of steel, while remaining less than the axial width of the radially outermost working ply, and in that the ratio of the rigidity R=(1/p)(dF/d.epsilon.) of extension per unit of width of the additional ply to the sum of the rigidities of all the other plies of the crown reinforcement ranges between 0.35 and 0.70, p being the pitch between the cables or reinforcing elements of the ply, F being the tensile force per cable or element, and .epsilon. the relative elongation equal to 0.5%.
Advantageously, from both the technical and the economic point of view, the value of this ratio will be obtained by using in the so-called additional ply metallic elements made of steel of a larger diameter than that of the metal cables made of steel forming the working plies.
"Inextensible cable" is to be understood to mean a cable, for instance a steel cable, which has a relative elongation of less than 0.5% measured at 25% of its breaking load.
Metallic elements oriented substantially parallel to the circumferential direction are elements which form angles within the range of +2.5.degree. to -2.5.degree. or around 0.degree. with said direction.
The rigidity of extension of a ply of reinforcing elements results from the tensile force exerted in the direction of the cables per unit of width of ply which is necessary to obtain a given relative elongation .epsilon., and may be expressed by the formula R=d/F/d.epsilon., R being the rigidity of the ply in question and dF/d.epsilon. the derivative of the tensile force per unit of width of the ply in relation to the relative elongation, or by the formula ##EQU1## p being the pitch between the elements of said ply, and dF/d.epsilon. the derivative of the tensile force per element in relation to the relative elongation.
In the above context, the additional ply may be formed of so-called semi-elastic continuous steel cables, that is to say cables having relative elongations upon rupture of between 2% and 6%. These cables make it possible to obtain the level of rigidity suitable for proper distribution of the circumferential tension between the working crown plies and the additional ply. Said cables are advantageously said to be "bimodular", that is to say, having a curve of tensile stress as a function of relative elongation having gradual slopes for the low elongations and a substantially constant, steep slope for the higher elongations. The very low modulus before curing for elongations of less than 2% permits an increase in the circumferential development of the additional ply during the curing of the tire.
The additional ply may also be formed of metal cables made of steel, oriented circumferentially and cut so as to form sections of a length very much less than the circumferential length of the ply, the cuts between sections being axially circumferentially offset relative to each other. Such an embodiment makes it possible, in simple manner, to impart to the additional ply the desired rigidity, whatever it may be.